The present invention relates to a self-recovering current-limiting device with liquid metal, including electrodes made of solid metal for the connection to an electric circuit to be protected and a plurality of compression spaces which are partially filled with liquid metal.
Soviet Union Patent Publication SU 922 911 A describes a self-recovering current-limiting device containing two electrodes made of solid metal which are separated by first insulating bodies which are designed as a pressure-resistant insulating housing. Inside the insulating housing, compression spaces are formed by insulating intermediate walls and second insulating bodies which are arranged therebetween and designed as ring-shaped sealing disks, the compression spaces being partially filled with liquid metal and arranged one behind the other and interconnected via connecting channels of the intermediate walls, the connecting channels being filled with liquid metal and arranged off-center. Thus, in normal operation, a continuous, inner conductive connection exists between the electrodes via the liquid metal. In the current-limiting event, the liquid metal is displaced from the connecting channels as a result of the high current density. In this manner, the electrical connection of the electrodes via the liquid metal is interrupted, resulting in the limiting of the short-circuit current. Subsequent to clearing or eliminating the short circuit, the connecting channels refill with liquid metal whereupon the current-limiting device is operational again. In German Patent Application DE 40 12 385 A1, a current-limiting device having only one compression space is described and vacuum, protective gas, or an insulating liquid are mentioned as the medium above the liquid level. It is known from German Patent Application DE 26 52 506 A1 to use gallium alloys, in particular GaInSn alloys as liquid metal in contact devices. Known, for example, from Japanese Patent Abstract JP 40 4312737 A are, current-limiting devices in which a tubular current-limiting chamber filled with liquid metal is in communication with a gas-filled, spring-loaded cylinder-piston device for absorbing the sudden pressure increase developing in the current-limiting event due to the vaporization of liquid metal.
To enable the current-limiting devices mentioned at the outset to be used for different cases of application, they have to be differently designed to the effect that they do not operate in response to an overloading of more or less short duration, depending on the case of application. Thus, for example, in conjunction with generators, current-limiting devices must not operate in response to a short-time overload current which is 2 . . . 6 times the nominal current or, in conjunction with motors of poor iron quality or transformers, in response to a short-time overload current which is 6 . . . 18 times the nominal current, but have to operate only in response to a current which in comparison is higher. Until now, therefore, there has been the disadvantageous requirement for the manufacturers to offer a considerable number of current-limiting devices to permit a suitable selection with respect to the conditions on the user side.
U. S. Pat. No. 4,429,295 describes a self-recovering device current limiting device containing hollow cylindrical electrodes made of solid metal, two compression spaces which are completely filled with liquid metal, and an intermediate wall which separates the compression spaces and which features connecting channels. The electrodes, together with in each case one inner piston made of insulating material, constitute a cylinder-piston device for taking up the evaporating pressure in the current-limiting event against a restoring means in the form of an inert gas or a spring means. British Patent GB 1 209 020 discloses a self-recovering current-limiting device in which a fixed electrode and a movable electrode are conductively connected via a reservoir which is completely filled with liquid metal and via a connecting channel. In the current-limiting event, the movable electrode is moved by the pressure of vaporizing liquid metal against a gaseous restoring means, it being possible for a plunger which is linked to the movable electrode and protrudes outward to be connected to an actuator for an indicating means or a circuit-breaker. After the current-limiting event has ended, the condensing liquid metal causes the movable electrode to return to the original position together with the plunger. The above described current-limiting devices are not suitable for an adjustment to a desired nominal current factor. Soviet Union Patent Publication SU 1 529 303 A describes an electric switching device with liquid metal, both of whose electrodes of solid metal, together with an insulating intermediate piece, constitute a receptacle whose inner surface is provided with sections of different diameter. By actuating the piston of a bellows filled with liquid metal, the liquid level inside the receptacle rises or decreases, as a result of which an electrical connection or disconnection takes place between the electrodes. The above described switching device is not suitable for limiting an overcurrent.
Therefore, an object of the present invention is to provide a current-limiting device which can be adapted to a desired current-limiting behavior, i.e., in particular with respect to the response, or minimum trip, current, in a manner which is easy to handle and reliably reproducible.
The present invention provides a self-recovering current-limiting device. The device includes a first and a second electrode for connection to an electric circuit to be protected, each of the first and second electrodes being made of a respective solid metal. A plurality of pressure-resistant insulating bodies and a plurality of insulating intermediate walls supported by the plurality of insulating bodies are provided. The plurality of insulating intermediate walls define a plurality of connecting channels therein and the plurality of insulating intermediate walls and the plurality of pressure-resistant insulating bodies at least partially define a plurality of compression spaces. The plurality of compression spaces are disposed one behind the other between the first and second electrodes and are at least partially filled with a liquid metal, a level of the liquid metal above the connecting channels being changeable using an adjusting device.
By an adjustment of the adjusting device, a change in volume of the liquid metal inside the compression spaces is effected, changing the filling level above the connecting channels. It was found that the magnitude of the response current increases with increasing filling level of the liquid metal above the connecting channels, provided that the remaining conditions are identical. In this manner, it is achieved for an individual device to be adjustable to a required nominal current factor of a plurality of possible nominal current factors on the user side. The current-limiting device is intended to operate in response to the nominal current, which the installation to be protected is designed for, multiplied by the nominal current factor.
In a first embodiment of the present invention at least one of the compression spaces is at least partially designed as a cylinder-piston device whose piston and, consequently, the filling level can be adjusted by an adjustable and arrestable adjusting device. Advantageous in this context is the conductive connection of the piston to the adjacent electrode, for example, via welded-on flexible conductors so that the piston acts like an inner electrode.
In a second embodiment of the present invention at least one of the compression spaces is connected to a reservoir which is filled with liquid metal, the reservoir being operatively connected to an adjusting device which can be adjusted and arrested from outside. Preferably, one of the electrodes is in communication with the reservoir. The reservoir is advantageously constituted by a cylinder-piston device or of a bellows.
The adjusting device is expediently connected to a plunger and/or can be arrested by way of arresting or clamping means and/or is provided with an adjusting scale, in this case advantageously in conjunction with a pointer.
If a plurality of current-limiting devices are combined to form a multipole device, then it is recommendable for the adjusting devices of all poles to be connected for jointly adjusting the same nominal current factor of the poles, for example, via a bridge.
GaInSn alloys as the liquid metal to be used are easy to handle because of their physiological harmlessness. An alloy of 660 parts by weight of gallium, 205 parts by weight of indium, and 135 parts by weight of tin is liquid from 10xc2x0 C. to 2000xc2x0 C. at normal pressure and possesses sufficient electrical conductivity.